Draft: bundle: add bundle verification options type

Bundle verification performed as part of unbundle() is configurable via providing verify_bundle_flags. This is done by invoking verify_bundle() and propagating the set flags. If the VERIFY_BUNDLE_FSCK flag is provided, the fsck-objects flag is specified when invoking git-index-pack(1) to perform fsck checks on the objects in the bundle.

Introduce a new type, verify_bundle_opts, and update unbundle() to accept this instead of verify_bundle_flags to perform the same verification configuration. In a subsequent commit, verify_bundle_opts will be extended to support configuration of fsck message severity.